LED display devices with arrays of LEDs have daylight display capabilities and are able to provide for relatively large displays that can be read from a distance at industrial sites.
Intrinsic safety (I.S.) is a design requirement in the art, used for electronic equipment for use at industrial sites such as oil terminals and mines, where normal operating conditions or spilling may give rise to the presence of inflammable or explosive gases. U.S. Pat. No. 7,312,716, for example discusses intrinsically safe designs of wireless communication network equipment. Intrinsically safe devices that use LEDs are used in U.S. Pat. No. 6,979,100, which involves intrinsically safe LED lighting, and U.S. Pat. No. 7,420,471, which uses a LED display to provide warning signals in a mine. As used herein, an intrinsically safe LED display is a LED display that is designed according to a requirement for intrinsic safety.
Research into possible causes of explosions has provided design rules for providing intrinsic safety. In some cases it is necessary to encapsulate electronic equipment to provide intrinsic safety. But it is also possible to provide intrinsic safety with equipment that has exposed components.
One important consideration for intrinsic safety is maximum component surface temperature. Research has shown that for most classes of explosive gases components with large surfaces that are exposed to gases from the environment are safe if their temperature remains below 135 degrees centigrade. For smaller surfaces, higher temperatures are allowed. Design requirements for intrinsic safety allow a temperature of 200 degrees centigrade for surfaces with an area of less than 2000 square millimetres, and if the area is that of a resistor, the design requirement for such areas is that the resistor dissipates less than 1.3 (in an ambient of no more than 40 degree centigrade, 1.1. Watt in ambients up to 80 degrees). For areas of less than 20 square millimetres a temperature of 275 degrees centigrade is allowed To provide for intrinsic safety, circuits should be designed so that these requirements are met both under normal operation and during conceivable malfunctions.
A conventional design solution to provide intrinsic safety is to put resistors in series with any circuit path that could be short-circuited due to malfunction, if the short-circuit could give rise to a temperatures above safe level. Such resistors serve to limit the dissipated power. Because a resistor will become the hottest point in the case of a short circuit of the protected circuit path, limitation by the resistor provides intrinsic safety without any dependence on proper operation of detectors, provided that the resistor does not dissipate so much power that it violates intrinsic safety requirements. The resistor values are typically chosen to limit power dissipation in the resistor to less than 1.1 Watt under normal and malfunction conditions. Usually resistors of less than 2000 square millimetre area are used. This means that the resistors temperature need not be limited to the 135 degree centigrade requirement that applies to large surface areas. It has been found that in an environment at less than 40 degrees centigrade a power dissipation from such resistors of no more than 1.3 Watt ensures intrinsically safe conditions (1.1 Watt in environments up to 80 Centigrade). In addition, power dissipation is kept below ⅔ of the power rating of the resistor to prevent that the risk of failure of the resistor exceeds an intrinsically safe level. Furthermore, intrinsically safe circuits use Zener barriers containing fuses in the safe area to limit the voltage, current and power supplied to such electronic circuits in the case of equipment failure, to ensure that the power levels never becomes sufficient to produce temperatures that give rise to an explosion risk.
It is desirable to provide for Intrinsically Safe LED display devices with a 2 dimensional array of LEDs, because of their daylight display capabilities and their ability to provide for relatively large displays that can be read from a distance at the industrial site (as used herein an array can be a matrix with rows and columns, but also other arrangements with rows of LEDs, such as a linear array with a single row of LED circuit cells, or 7 segment digit display arrangements, wherein the segments comprise rows of LEDs).
At the same time, it is desirable that the intrinsic safety of the LED display device should not prevent it from functioning as much as possible. For example, if the LED display device is used to indicate information that is needed to maintain safety in a mine or at an oil terminal, it is undesirable that more than a minimum number of LEDs or even the entire LED display device would switch off because some of its LEDs fail in a way that lead to a safety risk.
To provide for intrinsic safety of a LED in combination with continued operation, conventional protective series resistors may be used in series with individual circuit paths containing LEDs. However, this conventional approach does not provide for intrinsic safety in a LED display device wherein a large number of LEDs in parallel circuit paths is used in close proximity with each other. Intrinsic safety requires that such a display device cannot reach unsafe temperatures even if all LEDs short circuit simultaneously. When a plurality of mutually adjacent LEDs in a small area fails in this way, the maximum power available to the LED display device is dissipated in the protective current limiting resistors in the small area. It has been found that in this case the combined effect of the resistors can cause the maximum surface temperature to exceed the allowable limit, even if the power dissipated by each individual resistor remains below the safe value of 1.3 Watt (1.1 Watt in 80 degree environments).
EP 891 120 discloses the use of a PTC in series with a set of LED's to protect against destruction due to voltage rises. When the voltage rises, current increases, heating the PTC, which in turn leads to an increased resistance that reduces the current. EP 891 120 uses one PTC for a plurality of LEDs. The document does not discuss LED arrays in displays that have at least rows of LEDs that are fed from a power supply. But of course the LEDs of such a display could also be protected against voltage surges by a PTC. However, the document gives no reason to do so on a pixel by pixel basis for each pixel and of course there is no need to protect the LEDs if the power supply itself is designed to prevent voltage surges.
US 2007/139928 discloses the use of a PTC in series with a LED to protect against destruction due to excessive heating. The document does not discuss LED display arrays, with at least rows of LEDs that are fed from the same power source. The document gives no reason to protect LEDs on a pixel by pixel basis with different PTCs for each pixel and of course there is no need to protect the LEDs if the power supply itself is designed to prevent voltage surges.
CN 101 581 443 confirms that intrinsic safety has been considered for lighting devices that contain a single LED.